(1) Field of the Invention
The present invention is an acoustically transparent carbon nanotube transducer that can operate in a passive acoustic mode for data collection.
(2) Description of the Prior Art
The principle of thermal active acoustic transduction is that when alternating current is passed through a comparatively thin transducer; periodic heating takes place in the conductor following variations in power strength from an outside source. This periodic heating produces temperature waves which propagate into the surrounding medium. The amplitude of the temperature wave decreases rapidly as the distance from the conductor increases. Based on the rapid production of these temperature waves; the net effect is to produce a periodic rise in temperature in a limited portion of the medium near the conductor. Thermal expansion and contraction of this layer of the medium determines the amplitude of the resulting sound waves.
Recently, there has been development of underwater acoustic carbon nanotube (CNT) yarn sheets capable of producing high acoustic output at low frequencies with broad bandwidth. An underwater acoustic transmitter is feasible in which the transmitter uses thermal means of heated CNT substrates and in which a low frequency acoustic projector is formed. The acoustic carbon nanotubes can act as acoustic transducers while having a comparatively small volumetric size. The principle transduction for active acoustic carbon nanotubes is through thermal acoustics as opposed to conventional underwater transducers that utilize electromechanical vibrations.
In Osborn (U.S. Pat. No. 7,093,343), acoustic projectors are disclosed that generate seismic energy in marine geophysical testing. The acoustic projectors use energy comprising a pressure pulse that travels through the water and underlying subsurface geologic structures. The energy is partially reflected from interfaces between the geologic structure and is detected with geophone or hydrophone sensors. The cited reference includes five transducers but any number of transducers can be included.
Meisner (U.S. Pat. No. 7,427,201) discloses a current intersection point of carbon nanotubes that emit electrons which, in the vacuum space, excite and cause phosphor to emit light for display purposes. Active material includes a film of any active material or combination of active materials such as quartz, barium titanate, lead niobate, lead zirconate titanate, or piezo active plastic films such as KYNAR. These piezoelectric materials respond to external stimulus, such as sound waves which liberate localized electrons at the various junctions to create amplitude gains. Acoustical transducer arrays can reflect a sound signal in reverse to the sender which can be used for echo location devices.
In Jiang (U.S. Pat. No. 8,199,938), a method is disclosed for producing sound waves. The signal can be applied to the carbon nanotube structure by at least two electrodes from a signal device. When the signals are applied to the carbon nanotube structure; heating is produced in the structure according to variations of the signals. The carbon nanotube structure transfers heat to a medium in response to the signal and the heating of the medium causes thermal expansion of the medium. It is the cycle of relative heating which results in sound wave generation.
In Jiang (U.S. Pat. No. 8,494,187) a sound wave generator is disclosed which includes a carbon nanotube structure and an insulating reinforcement structure in which both constitute a free-standing structure. When holding a point of the carbon nanotube structure, the entire structure can be lifted without being destroyed. The carbon nanotube structure includes a plurality of carbon nanotubes joined by a van der Waals attractive force therebetween.
In Jiang (U.S. Pat. No. 8,537,640), a carbon nanotube film structure of an acoustic element includes at least two stacked carbon nanotube films. In other embodiments, the carbon nanotube structure can include two or more coplanar carbon nanotube films and can include layers of carbon nanotube films. Additionally, when the carbon nanotubes in the carbon nanotube film are aligned along one preferred orientation; an angle can exist between the orientations of carbon nanotubes in adjacent films, whether stacked or adjacent.
Wang (U.S. Pat. No. 8,553,912) discloses a sound wave generator that includes a carbon nanotube structure. The structure can include a plurality of carbon nanotubes uniformly distributed therein. The carbon nanotubes can be combined by a van der Waals attractive force therebetween. The carbon nanotubes in the structure can be selected from single-walled, double-walled or multi-walled carbon nanotubes. There may be many layers of ordered and/or disordered carbon nanotube films in the structure.
Liu (U.S. Pat. No. 8,811,631) discloses a thermo-acoustic device that includes an electrode layer and a sound wave generator. The sound wave generator is disposed on a surface of the electrode layer. The electrode layer includes a plurality of insulated wires and a plurality of conductive wires that are weaved together to form a net structure and an electrode layer as an intertexture. The sound generator includes a carbon nanotube structure with a plurality of nanotubes oriented in the same direction.
Jiang et. al (U.S. Pat. No. 8,958,579) discloses a signal input device that can be a light source for generating light signals. The light signals can be directly transferred to the sound wave generator and a thermos-acoustic device works under a photo-acoustic effect. The photo-acoustic effect is a kind of thermo-acoustic effect and a conversion between light and acoustic signals due to absorption and localized thermal excitation. When rapid pulses of light are incident on a sample of matter; the light can be absorbed and the resulting energy will then radiate as heat. The heat causes detectable sound signals due to pressure variation in the surrounding medium. The thermo-acoustic device includes a signal input device, a sound wave generator and a substrate in a composite carbon nanotube structure.
Wei et. al (U.S. Pat. No. 9,061,906) discloses that films in a carbon nanotube structure can be co-planar or stacked. The number of layers of the carbon nanotube films is not limited. However, as the stacked number of the carbon nanotube films increases; the specific surface area of the carbon nanotube structure will decrease. Stacking the carbon nanotube films adds to the structural integrity of the carbon nanotube structure.
Schaedler et. al. (U.S. Pat. No. 9,217,084) discloses embodiments that include allowing fluids to be included in the region containing the aligned CNT layer with a bias force that optionally may be included perpendicular to the plane of the CNT array. The bias increases the compression of the CNT material and allows for more displacement parallel to the array.
Aliev et. al. (United States Patent Publication No. 2016/0037267) discloses a typical structure of an encapsulated active thermos-acoustic effect device. The device has two conductive electrodes attached to opposite edges of a vibrating plate thru the elastic silicon rubber. The thin carbon nanotube sheet (or a plurality of sheets superimposed on each other) suspended between two plates is connected to electrodes. The interior of the thereby assembled encapsulated device is filled with inert gas.
The optionally multilayered carbon nanotube sheet strips are superimposed on each other under a small angle to the nanotube alignment direction to enhance electrical conductivity in the perpendicular direction. A suspended part of a carbon nanotube sheet will create a temperature gradient that alternates at a sound frequency.
In the above active acoustic transductions; the electric power (of voltage squared over impedance, not the voltage itself) is proportional to temperature deviation. The temperature is directly proportional to a pressure disturbance wave. That is, if the driving voltage is at a frequency of ω, the electric power will be at a frequency of 2ω. The frequencies from temperature deviation and acoustic waves are both at 2ω as well.
Here, the energy flow is from electric to Joule heat, which is an irreversible process in that a reciprocal thermo-acoustic process becomes impossible by any active thermo-acoustic device. This process is similar to the energy transformation from electrical to light (with irreversible Joule heat) by an electric light bulb. The reciprocal (or reverse) of this process by supplying heat and shining light to the same electric bulb in order to generate electrical energy back is impossible, simply because Joule heat is involved.
Based on the cited references and the Joules heat effect by known active carbon nanotube transducers; there is a need for an alternate use of carbon nanotube transducers. As such, there is a use for a carbon nanotube transducer to operate in a passive mode. The inventive passive transducer could then be used as an underwater acoustic transduction hydrophone.